As new material such as ceramics are gaining acceptance in industry, it becomes critical to develop inspection methods for evaluation of their surface characteristics. One of the potential applications of ceramic materials is for balls for ball bearings. Balls in such bearings are subject to high dynamic stress and due to the brittleness of the ceramics it is important to inspect each ball and be able to detect surface and subsurface cracks ten micrometers in size, or smaller. It is essential to be able to detect such cracks using a fast system that allows full coverage of the bearing.
There are basically three types of surface defects. The first type are the fissures that exist near the surface and perpendicular to it. These cracks can easily be detected using surface acoustic waves which are very sensitive to this kind of perturbation.
The second type of defects are surface gouges which are shallow but relatively large, typically 10 micrometers or more in size. These are once again very easily detected by measuring the phase of the acoustic signal reflected from the bearing at the location of the defect. For an operating frequency of 120 MHz, where the wavelength in water is about 12 micrometers, one is able to image these defects even if their depths do not exceed 0.5 micrometers which corresponds to a phase variation of 30.degree., and without particular precautions concerning scanner vibrations.
The last kind of defect includes shallow cracks of small width with smooth corners. They are the most difficult to detect because they are insensitive to surface wave and are small compared to the spot size of the focused acoustic beam. Their size can be in the order of one micrometer while the beam size in the 100 MHz range is at least an order of magnitude larger.